Dick Johnson methodology for measuring glider performance

Originally Posted by

I guess people found out about the extreme loss of performance of a
wet or dirty LS-3 and spreat the word...


Cheers
Andreas

Indeed, many gliders from that era that used the same family of airfoils suffered thusly, perhaps none so reputed as the PIK 20. But I think the impact here in the U.S. was more from occasional rain than from the bugs that seem more a problem in Europe. Those of us who campaigned LS-3s here often left the wings sanded to a satin finish--without wax--so the rain would spread rather than bead up, which seemed to help significantly. 400 grit seemed about right although one top pilot left his at 220 grit--and joked about having to sand the occasional bugs off each day.

Just before I sold mine, I contoured the wings and polished them up, then flew a last contest that took me into rain one day. Nothing evil happened. I'd since heard from a PIK driver that using more positive flap than usual and flying a bit slower helped cure the "falling out of the sky" phenomenon for which PIKs were infamous. It seemed to work well for the LS-3, too.

To the question of CG position, I flew mine with the CG at the aft end of the range. That seemed to yield the best performance, the only negative being somewhat more pitch sensitivity in rough thermals.

Chip Bearden
ASW 24 "JB"

Hi

FWIW I used to have a share in a Janus and I too found that having some extra positive of flap and flying a little slower helped to avoid the plummeting when wet. No hard figures to verify that but the airframe shaking went away as well so more comfortable for the poor old pilot.

:-) Colin

wrote on 7/12/2016 8:26 AM:
I don't disagree with this post but I would say that, overall, DJ's
testing was good at pointing out the gliders that performed
particularly well compared with its competitors - and this was often
confirmed by the choices of top competition pilots. Example: ASW20
versus Mini Nimbus and LS3a.

One of the flaws in DJ's method (and the Akafliegs) is it doesn't test
the performance dynamically; ie, while dolphin soaring or flying in
turbulent air and thermals.

I once noticed the superior dolphin ability of my ASW 20 vs a Venus
while we were under clouds, quickly gaining on him. As soon as we
started a 10 mile glide in the blue, he slowly rose above me, over 800'
or so by the time we got to the next cloud.

The early ASW24, and others with that generation of airfoils (like my
ASH 26E), showed the problems induced by a "flat spot" in the L/D curve
when the air isn't smooth.

For competition purposes, I suggest competitions are the best test for a
glider. For recreational purposes, ultimate performance is unimportant
compared the other features of the glider.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

ZZ wrote on 7/12/2016 9:00 PM:
What caught me by surprise was his glide calculations used computed
TRUE SINK as well. This might have made sense to me if he was
measuring vertical speed with a rate instrument but unless I am
mistaken, he measured sink rate using an altimeter and a stop watch.

Since the altimeter (properly corrected) would give true altitude loss,
and the stop watch would give true duration, I think (altitude
loss)/(duration) would give true sink rate.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf

I was fortunate to have assisted DJ in testing several gliders and he/we did measure sink rate by timing descents. Typically in 1,000 ft increments. Also, we/he did several flights for critical data points and threw out data from flights that obviously suffered from atmospheric or data anomalies. Data points from the remaining flights were manually averaged to create the finished polar. And, he did normalize the data to sea level, standard atmosphere, and TAS conditions. There was at least one article in Soaring in which he published the formulas. Interestingly, when I first analyzed data and created graphs using Lotus 1-2-3 (does that age me or what??), Dick suggested I was wasting my time as he trusted calculators and graphs created manually more. I'm pleased to see he eventually accepted Excel outputs.

Thank you for your reply. Fortunately couple of very helpful individuals supplied virtually all the info I needed and I was able to do my tests. Despite few individuals casting doubts on the methodology, I was able to get enough data points that allowed to get a line of best fit with all the points pretty much on that line. Also the sink rates at relevant speeds matched factory sink rates. An interesting exercise anyway.

Paul